Influenza and vaccinia viruses expressing malaria CD8⁺ T and B cell epitopes: Comparison of their immunogenicity and capacity to induce protective immunity

We compared the effectiveness of several recombinant influenza and vaccinia viruses to induce a malaria-specific immune response. The CD8⁺ T cell epitope of the circumsporozoite (CS) protein of Plasmodium yoelii, a rodent malaria parasite, was expressed in two distinct influenza virus proteins, the hemagglutinin and the neuraminidase. These recombinant viruses were found to be equally efficient at inducing CS-specific CD8⁺ T cells in mice. A third recombinant virus, which expresses a B cell epitope of the CS protein, induced neutralizing anti-sporozoite Abs. Expression in the same recombinant virus of the CD8⁺ T cell epitope and of the B cell epitope did not impair the capacity of this recombinant virus to induce malaria-specific CD8⁺ T cells and neutralizing Abs. The immunogenicity of a vaccinia virus, expressing the entire CS protein, was compared with that of a highly attenuated vaccinia strain expressing the same protein and with that of another vaccinia virus expressing only the CD8⁺ T cell epitope. All three vaccinia virus recombinants elicited CS-specific CD8⁺ cells and a potent inhibitory response against pre-erythrocytic stages of malaria parasites. Optimal levels of anti-sporozoite Abs, inhibition of liver stage development, and protection against malaria infection resulted from repeatedly immunizing the animals with recombinant influenza viruses followed by boosters with a recombinant vaccinia virus. These findings support the concept that live viral vectors expressing the appropriate proteins and/or epitopes can be used as promising vaccine candidates.